Transverse flow laser cell

ABSTRACT

A laser cell for a transverse flow liquid laser has an active region in the form of a rectangular prism and cylindrical input and output chambers mounted in spaced-apart relationship in the transverse direction at opposite ends of the active region. A baffle positioned in the input chamber causes the liquid to flow uniformly through the active region.

[45] July 18, 1972 United States Patent Kocher et al.

[56] References Cited UNITED STATES PATENTS [54] TRANSVERSE FLOW LASERCELL [72] Inventors: Robert C. Kocher, Great Neck; Franklin K. Moore,Ithaca; Harold Samelson, Sea Cliff; William R. Watson, College Point,all of NY.

3,302,127 1/1967 Shao-chiLin.....................i...331/94.5 3,571,7473/1971 Bronfinetal.........................331/945 PrimaryExaminer-William L. Sikes Attorney-Irving M. Kriegsman [73] Assignee:GTE Laboratories Incorporated [221 Filed: Nov. 24, 1970 [21] Appl. No.:92,354

ABSTRACT A laser cell for a transverse flow liquid laser has an activeregion in the form of a rectangular prism and cylindrical input andoutput chambers mounted in spaced-apart relationship in the transversedirection at opposite ends of the active region. A bafile positioned inthe input chamber causes the liquid to flow uniformly through the activeregion.

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[52] U.S. [51] Int. [58] Field of 7 Claim, 3 Drawing FiguresPATENTEUJULWIQTZ 3,678,410

7' '7 30 HEAT 34 EXCHANGER ROBERT KOCHER FRANKLIN MOORE, HAROLD SAMELSONWILLIAM R. WATSON TRANSVERSE FLOW LASER CELL BACKGROUND OF THE INVENTIONThis invention relates to lasers and, in particular, to cells for laserswhich utilize a flowing liquid as the active material.

Solids, gases and liquids have been used as active materials in lasers.Lasing is initiated by raising the energy levels of the atoms in theactive material from the levels which they normally occupy to a higherenergy level or excited state. This process of excitation is generallyaccomplished in a liquid laser by a high intensity light source. Ashereinafter used, the term active material shall refer only to a liquidactive material.

When the atoms of an active material are in an excited state, they canbe stimulated to emit a photon by interaction with an incident photon.As a result, the incoming photon, or wave, is augmented by the one givenup by the excited atom. The released wave falls in phase with the wavethat triggered its release. Hence, an amplifying action ensues.

The active material is excited while in a laser cell. In order tosustain laser operation, the laser cell must be part of a resonantcavity having at least two separated reflecting surfaces, one of whichis partially transmissive. The wave caused by the release of the photonfrom the atom in the excited material must travel a path which issubstantially parallel to the longitudinal axis of the resonant-cavityso that it may be repeatedly reflected through the active material. Asthe wave travels through the material, it stimulates more atoms torelease photons which further amplifies and reinforces the wave. Eachtime the wave is reflected at the partially transmissive reflectingsurface a small portion of it passes through the surface. This smallportion constitutes the laser output beam.

Two desirable properties of the laser output beam are coherence andcollimation. Optical distortion of the laser output beam shall herein betaken to mean an impairment of either of these properties. The opticalcharacteristic of liquid active material which primarily determines thequality of the output laser beam is the uniformity of the refractiveindex of the liquid which, in turn, is dependent upon the uniformity ofthe liquids temperature and density. When a laser is excited, largeamounts of heat may be absorbed unevenly by the liquid tending to causeit to have warmer and cooler layers which results in a non-uniformrefractive index. Excitation of the laser while these non-uniformconditions exist may cause optical distortion of the laser output beam.

Optical distortion of the output beam may be prevented by insuring thatisothermal conditions exist in the cell prior to excitation. The flowingliquid laser, wherein the liquid active material continuously flowsthrough the laser cell, was developed to provide a liquid laser capableof frequent, successive excitations. Isothermal conditions areestablished in the cell of the flowing liquid laser by causing themixing of adjacent liquid layers thereby reducing temperatureinhomogeneities and index of refraction gradients in the liquid prior toexcitation and replacing the excited liquid by unexcited liquid betweensuccessive excitations. The time between successive excitations, i.e.the pulse repetition rate, therefore is dependent upon the linear rateat which the liquid material flows through the active region. Priorcells for liquid lasers required relatively high linear liquid flowrates to obtain high pulse repetition rates. The cell of this inventionachieves high pulse repetition rates at relatively low linear liquidflow rates.

7 SUMMARY OF THE INVENTION The present invention relates to a cell for alaser and, in particular, to a cell for a flowing liquid laser whereinthe liquid flows through the cell in a direction transverse to the laseroutput beam.

Excitation of the flowing liquid laser takes place along thelongitudinal axis of the cell at a place where the liquid flow isuniform and isothermal. Since the liquid flows in the transversedirection, each excitation produces an excited volume of the lasermaterial extending along the entire longitudinal axis of the cell andextending a relatively small distance on either side of that axis in thetransverse direction. This excited volume of liquid has temperature andindex of refraction gradients; therefore it is desirable to replace thisentire volume of excited liquid with unexcited isothermal activematerial prior to the next laser excitation. Since the liquid flowstransverse to the excitation and only a relatively small transverseportion of the liquid is excited, the entire volume of excited liquidmust flow only a relatively small transverse distance before a newisothermal volume of liquid becomes available for excitation. In thismanner, a relatively high pulse repetition rate is achieved with arelatively low linear flow rate.

While relatively high pulse repetition rates are achievable with atransverse cell, it is difiicult to achieve uniform flow conditions inthe excitation region of the cell. In a transverse.

cell, liquid active material flows into the cell from a cylindrical pipecoupled to the cell. Generally, the volume of the cell is relativelylarger than that of the pipe with the cell extending in a directionperpendicular to the liquid flow. The liquid active material thereforetends to flow through the center of the cell with only a relativelysmall portion of the liquid flowing proximate to the cell walls. Theunequal flow distribution results in the establishment of a velocitygradient within the cell with the liquid at the center of the cellflowing relatively faster than the liquid near the cell walls. Due tothe velocity gradient, excitation of the cell gives rise to temperatureand index of refraction gradients within the liquid active materialwhich causes distortion of the laser output beam. The cell of thisinvention produces uniform flow of liquid in the excitation region byproviding a transition area between the entrance to the cell and theexcitation region wherein the liquid flow is smoothed.

The cell comprises an active region having a longitudinal and atransverse axis and an input and an output chamber coupled to the activeregion in spaced apart relationship along the transverse axis. Bafflemeans is positioned in the input chamber. Liquid active material flowinginto the input chamber of the cell is incident on the bafile means whichpromotes the mixing of the active material as it flows through the inputchamber of the cell and produces uniform liquid flow in the activeregion of the cell.

In one embodiment of the invention the baffle means comprises a platehaving a plurality of holes which is positioned in the input chamber andextends in the longitudinal direction for the entire length of the inputchamber thereby dividing the input chamber into two sections. Liquidactive material flows into the first section wherein it is incident onthe plate which nearly stops the flow of liquid and causes the liquid tobe retained in the first section long enough for the liquid to becomeuniformly distributed with nearly zero velocity and uniform pressureadjacent the entire upstream surface of the plate. In addition, liquidactive material in the first section upstream from the plate isthoroughly mixed thereby eliminating inhomogeneities which may exist inthe liquid active material flowing into the input chamber.

The liquid active material which is distributed over the upstreamsurface of the plate is then forced to flow through the holes in theplate thereby producing a plurality of uniformly distributed identicalsmall liquid jets adjacent the downstream surface of the plate. Theseliquid jets interact to produce mixing of the liquid active material.Since the average flow rate of the downstream side of the plate equalsthe average flow rate at the upstream side, i.e. approximately zero,mixing of the liquid jets occurs at a relatively low transverse flowspeed. Thus, a uniform slowly flowing liquid mass is produced adjacentthe downstream side of the plate. The uniform liquid flows into theactive region wherein it is excited to produce a laser outputhavingrelatively low distortion.

Further features and advantages of the invention will become morereadily apparent from the following detailed description of a specificembodiment of the invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a representation of aflowing liquid laser.

FIG. 2 is a cross-sectional view of the cell ofthis invention.

FIG. 3 is an isometric representation of the cell of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a schematic representation of the circulating liquid laser. Acirculating liquid laser is one in which the liquid active materialcontinuously flows through a closed system. Although many liquid activemedia for lasers are known and can be used, in the preferred form ofthis invention the liquid active medium is a solution of a dye, asolvent and a triple state quencher. Preferably, the dye is Rhodamine6G, the solvent is ethanol, and the triple state quencher iscyclooctatetraene, hereinafter referred to as COT. This solution may beprepared by stirring a measured amount of Rhodamine 6G in ethanol untilthe dye is fully dissolved. COT is then added to the dye solution.

Referring now to FIG. 1, the circulating liquid laser comprises a cell10 having a longitudinal axis 12 perpendicular to the plane of FIG. 1and a transverse axis 14. Mirror 16 is mounted externally to the cell.Alternatively, mirror 16 may be a reflective coat placed directly on thecell wall. A second mirror (not shown) is positioned in opticalalignment with mirror 16 at the other side of the cell. One of themirrors, for example mirror 16, a totally reflecting, i.e. it reflectssubstantially all the light impinging upon it. The other mirror is onlypartially reflective in that it reflects only portion of the lightimpinging upon it while permitting the remainder of the light to betransmitted therethrough as the output beam of the laser.

The cell further comprises an active region 20 in the form of arectangular prism, an input chamber 22 and an output chamber 24positioned at opposite ends of active region 20. The input and outputchambers are in the form of hollow elon' gated cylinders extending inthe longitudinal direction.

Excitation means in the form of light source 26 is positioned in closeproximity to active region 20 and extends along longitudinal axis 12.The light source, which may be a high pressure Xenon filled lampcontrolled by an external circuit (not shown) provides a source ofexcitation energy to the active material flowing through the cell. Thewalls of active region 20 are constructed of quartz or other materialwhich transmits radiation with negligible attenuation at the frequencyrequired to excite the active material.

The laser further comprises a pump 28 for circulating the liquid activematerial and a heat exchanger 30 for cooling the liquid after it flowsout of the cell. Lines 32, 34 and 36 interconnect pump 28, heatexchanger 30, and cell 10.

In operation, pump 28 circulates the liquid active material through theinput chamber 22, active region 20 and output chamber 24 of cell 10. Thelaser output beam is obtained by actuating light source 26. Part of theenergy produced by the light source is absorbed in the liquid activematerial in the active region thereby producing a volume of excitedliquid in the form of a rectangular prism proximate to the light sourceand extending in the longitudinal direction over the entire width of theactive region and for a relatively small distance in the transversedirection. The excited volume of liquid contains photons which arerepeatedly reflected through the active medium by the mirrors therebyproducing the laser output beam. Maximum power output and minimumdistortion of the laser output beam are obtained when the liquid activematerial is uniform in the transverse and longitudinal directions justprior to excitation and the rectangular prism of liquid occupics theentire active region with the sides of the prism parallel to thecorresponding walls of the active region. Cell 10 is designed to providethese conditions.

Referring to FIGS. 2 and 3, input chamber 22 has positioned therein abaffle comprising a plate 38 having a plurality of holes 39. The plateextends in the longitudinal direction for the entire length of the inputchamber thereby dividing the input chamber into a first section 40 and asecond section 42. Liquid active material flows into first section 40 ofthe input chamber through input port 44. Since the liquid volume cannotexpand, the liquid tends to flow directly into the first section and isincident on plate 38 in an area opposite input port 44. The plate nearlystops the liquid flow causing the liquid to be retained in first section40 upstream of the plate long enough to ensure that mixing eliminatesinhomogeneities which may be present in the liquid prior to entry intothe cell and producing a uniform condition of the liquid throughout thevolume of first section 40. Thus, the liquid active material becomesuniformly distributed over the upstream surface of plate 38 with anaverage flow rate approximately equal to zero adjacent the plate.

The liquid active material is then forced by the action of the pump 28through the holes in the plate 38 into second section 42 of inputchamber 22 and emerges on the downstream side of the plate as aplurality of identical liquid jets. These high speed jets of liquidemerging from the plate interact to ensure thorough mixing of theliquid. Since the total volume rates of liquid flow adjacent theupstream and downstream sides of plate 38 must be equal, and the totalflow area is unchanged in crossing the plate, mixing of the liquid jetsoccurs at the same very low average transverse flow rate as exists inthe first section 40. Therefore, mixing of the liquid occurs adjacent toplate 38 and a uniform liquid is produced in second section 42relatively close to the plate. As the liquid continues to flowdownstream towards the active region, the liquid flow rate increases asthe volume of section 42 decreases and a uniform flow of liquid isprovided in the active region. Although only one plate is shown in inputchamber 22 the input chamber may contain a plurality of similar platesto control the liquid flow.

After the liquid flows out of active region 20 it enters output chamber24. The output chamber may comprise a second plate 48 similar to plate38 extending along the longitudinal direction over the entire length ofoutput chamber 24 and dividing the output chamber into a first section50 and a second section 52. The output chamber prevents stagnation ofthe liquid active material in the active region of the cell and assuresa smooth uniformly accelerated liquid flow out of the active region.

Light source 26 can be positioned along longitudinal axis 14 at thepoint where the most uniform liquid flow conditions exist. This pointcan be ascertained by establishing steady flow conditions in the lasersystem and then probing the active region of the cell along thelongitudinal axis with a second laser beam. The point where the secondlaser beam experiences minimum distortion is where uniform flowconditions exist. Light source 26 may thereafter be positioned at thispoint.

Cell 10 is particularly advantageous when a high pulse repetition rateor continuous wave (cw) operation is desired. In a liquid laser mediumafter excitation, the volume of excited liquid will contain temperaturegradients giving rise to density and index of refraction gradients. Toobtain minimum distortion of the laser output beam, this volume ofexcited liquid should be replaced by unexcited liquid prior to the nextsucceeding laser excitation. Since in this transverse flow cell only arelatively small amount of liquid is excited in the transversedirection, the volume of excited liquid need be displaced only arelatively short transverse distance before being replaced by anunexcited volume of liquid. The entire volume of excited liquid cantherefore be quickly replaced with unexcited liquid and reexcitation ofthe laser can occur rapidly.

In a typical cell the inside diameter of the inputand output chambersare about 1 inches and the chambers extend about 2 V2 inches in thelongitudinal direction. Plates 38 and 48 contain 35 holes symmetricallyplaced and the plates are 5/64 inches thick. The active region extends 2inches in the transverse direction, 4 inches in the longitudinaldirection, and is 3/16 inches wide.

What is claimed is:

l. A cell for a laser utilizing a flowing liquid as active material,comprising:

a. an active region having a longitudinal axis and a transverse axis;

b. an input chamber coupled to said active region having a port thereinfor receiving the flow of active material, said active material flowinginto said input chamber parallel to said transverse axis; and

c. baffle means positioned in said input chamber cooperatively with theincoming flow of active material, said baffle means promoting the mixingof said active material in said input chamber, which causes a uniformflow of liquid active material in said active region.

2. The cell of claim 1, wherein said baffle means comprises a platehaving a plurality of holes therein, said plate extending .,,in thelongitudinal direction for the entire length of said chamber anddividing said input chamber into first and second sections, said liquidactive material flowing into said cell being incident on said platecausing said liquid active material to slow down and be retained in saidfirst section, said liquid active material becoming uniformlydistributed over the surface of said plate, said liquid flowing throughthe holes in said plate and emerging into the second section as aplurality of liquid jets, said liquid jets interacting to produce mixingof the liquid causing uniform liquid flow in the second section of saidinput chamber.

3. The cell of claim 2, wherein said input chamber is in the form of ahollow cylindrical body having a longitudinal axis parallel to thelongitudinal axis of said active region.

4. The cell of claim 3, wherein said active region is in the form of arectangular prism.

5. The cell of claim 4 further comprising an output chamber coupled tosaid active region in spaced apart relationship along said transverseaxis from said input chamber for receiving the liquid which flows out ofsaid active region.

6. The cell of claim 5, wherein said output chamber comprises a platehaving a plurality of holes therein, said plate extending in thelongitudinal direction for the entire length of said output chamberthereby dividing said output chamber into two sections.

7. The cell of claim 6, wherein said output chamber is in the form of ahollow cylindrical body having a longitudinal axis parallel to thelongitudinal axis ofsaid active region.

1. A cell for a laser utilizing a flowing liquid as active material,comprising: a. an active region having a longitudinal axis and atransverse axis; b. an input chamber coupled to said active regionhaving a port therein for receiving the flow of active material, saidactive material flowing into said input chamber parallel to saidtransverse axis; and c. baffle means positioned in said input chambercooperatively with the incoming flow of active material, said bafflemeans promoting the mixing of said active material in said inputchamber, which causes a uniform flow of liquid active material in saidactive region.
 2. The cell of claim 1, wherein said baffle meanscomprises a plate having a plurality of holes therein, said plateextending in the longitudinal direction for the entire length of saidchamber and dividing said input chamber into first and second sections,said liquid active material flowing into said cell being incident onsaid plate causing said liquid active material to slow down and beretained in said first section, said liquid active material becominguniformly distributed over the surface of said plate, said liquidflowing through the holes in said plate and emerging into the secondsection as a plurality of liquid jets, said liquid jets interacting toproduce mixing of the liquid causing uniform liquid flow in the secondsection of said input chamber.
 3. The cell of claim 2, wherein saidinput chamber is in the form of a hollow cylindrical body having alongitudinal axis parallel to the longitudinal axis of said activeregion.
 4. The cell of claim 3, wherein said active region is in theform of a rectangular prism.
 5. The cell of claim 4 further comprisingan output chamber coupled to said active region in spaced apartrelationship along said transverse axis from said input chamber forreceiving the liquid which flows out of said active region.
 6. The cellof claim 5, wherein said output chamber comprises a plate having aplurality of holes therein, said plate extending in the longitudinaldirection for the entire length of said output chamber thereby dividingsaid output chamber into two sections.
 7. The cell of claim 6, whereinsaid output chamber is in the form of a hollow cylindrical body having alongitudinal axis parallel to the longitudinal axis of said activeregion.